The CO-H 2 Conversion Factor in Galaxies Desika Narayanan Bart J - PowerPoint PPT Presentation

The CO-H 2 Conversion Factor in Galaxies Desika Narayanan Bart J Bok Fellow University of Arizona (With: Mark Krumholz, Eve Ostriker, Lars Hernquist)

H 2 CI CI CI H 2 H 2 CO H 2 H 2 H 2 CO CI CO H 2 H 2 CO H 2 H 2 H 2 H 2 CI

H 2 CI CI CI H 2 H 2 CO H 2 H 2 H 2 CO CI CO H 2 H 2 CO H 2 H 2 H 2 H 2 CI 1. Assume GMC is viralized and use CO line width as mass measurement

H 2 CI CI CI H 2 H 2 CO H 2 H 2 H 2 CO CI CO H 2 H 2 CO H 2 H 2 H 2 H 2 CI 1. Assume GMC is viralized and use CO line width as mass measurement II. Assume a DTG ratio and get dust masses

H 2 CI CI CI H 2 H 2 CO H 2 H 2 H 2 CO CI CO H 2 H 2 CO H 2 H 2 H 2 H 2 CI 1. Assume GMC is viralized and use CO line width as mass measurement II. Assume a DTG ratio and get dust masses III. CR + H 2 --> γ -ray

H 2 CI CI CI H 2 H 2 CO H 2 H 2 H 2 CO CI CO H 2 H 2 CO H 2 H 2 H 2 H 2 CI 1. Assume GMC is viralized and use CO line width as mass measurement II. Assume a DTG ratio and get dust masses III. CR + H 2 --> γ -ray Xco = N H2 /I co = 2-4 x 10 20 cm -2 /K-km s -1

Xco is Similar for Local Group Blitz et al., PPV Review Article, 2006

Xco = N H2 /I CO Depends on Galactic Environment “MW Xco” Tacconi et al. 2008 Desika Narayanan

Xco = N H2 /I CO Depends on Galactic Environment: High Surface Densities “MW Xco” “ULIRG Xco” Tacconi et al. 2008 Desika Narayanan

Xco = N H2 /I CO Depends on Galactic Environment: Low Metallicities CI Leroy et al. 2011 Genzel et al. 2011 (local galaxies) ( z ~1)

What’s at Stake KS Relations and Star Formation Efficiencies Molecular to Atomic Gas Mass Ratios Leroy et al. 2009 Saintonge et al. 2011 Lagos et al. 2011 Saintonge et al. 2011 Genzel et al. 2010 Daddi et al. 2010 CO Luminosity Functions and Ω H2 (z) Lagos et al. 2011 Obreschkow & Rawlings 2009 Keres, Yun & Young 2003 Desika Narayanan

Gadget: to get model discs and Sunrise: to get dust mergers at z=0,2 temperatures Jonsson et al. 2006, 2009 Springel et al. 2003-2005 Jonsson & Primack 2010

What do the molecules look like? -H2-HI balance calculated by balancing growth of H2 on grains with LW band photodissociation (Krumholz, McKee, Tumlinson 2010) CO -CO-CI balance function of ISRF, Z (Wolfire et al. 2010) CI H2 HI -Temp calculated by balancing PE, CR heating, line cooling and thermal exchange with dust (Krumholz, Leroy, McKee 2011; Juvela 2011) -GMCs isothermal, constant density spheres with floor surface density of ~10 22 cm -3 -Monte Carlo code: Calculates full statistical equilibrium of level populations in a 3D velocity, temp, density field within GMCs and galaxies (DN+2008, Krumholz & Thompson 2007, DN+2011) Desika Narayanan

Xco in Discs and Mergers Narayanan, Krumholz, Ostriker & Hernquist 2011 Desika Narayanan

Xco = N H2 /I CO ~ N H2 /(T* σ ) I ~T b ~T k T b σ velocity Desika Narayanan

Xco = N H2 /I CO ~ N H2 /(T* σ ) 10 22 cm -3 Narayanan, Krumholz, Ostriker & Hernquist 2011

Xco = N H2 /I CO ~ N H2 /(T* σ ) 5 km/s 10 22 cm -3 Narayanan, Krumholz, Ostriker & Hernquist 2011

Xco = N H2 /I CO ~ N H2 /(T* σ ) 5 km/s 10 22 cm -3 10 22 cm -3 10-20 K X co (MW) = few x 10 20 cm -2 /K-km/s Narayanan, Krumholz, Ostriker & Hernquist 2011

Xco = N H2 /I CO ~ N H2 /(T* σ ) 10 22 cm -3 10 23 cm -3 Narayanan, Krumholz, Ostriker & Hernquist 2011

Xco = N H2 /I CO ~ N H2 /(T* σ ) 50 km/s 10 22 cm -3 10 23 cm -3 Narayanan, Krumholz, Ostriker & Hernquist 2011

Xco = N H2 /I CO ~ N H2 /(T* σ ) 50 km/s 10 22 cm -3 10 23 cm -3 50 K X co (MW) = few x 10 19 cm -2 /K-km/s Narayanan, Krumholz, Ostriker & Hernquist 2011

Xco decreases with increasing ∑ H2

Xco increases with decreasing Z CO H 2 CI H 2 H 2 CO H 2 CI H 2 H 2 CI H 2 CO H 2 H 2 H 2 CI CI CI H 2

A General Prediction for Xco Xco ~ Σ H2 -0.2 e -Z/Z ❂

A General Prediction for Xco Xco ~ Σ H2 -0.2 e -Z/Z ❂

Conclusions Xco a continuous function dependent on metallicity and thermal and dynamical state of galaxies - In starburst galaxies hotter and high velocity dispersion gas causes Xco (on average) to be lower than Galactic mean - In low metallicity galaxies, lack of dust shielding increases mass of CO-dark clouds, and drives Xco to larger values than Galactic mean Desika Narayanan